Storage and transportation of liquefied natural gas is an extremely unique field, where most general metal containers cannot meet the requirements for tanks for storing liquefied natural gas. Austenitic stainless steels present good characteristics in terms of flexibility and sturdiness at low temperatures, and additionally, are fairly compatible with natural gas. As such, such steels are often chosen as the material of an internal tank. However, austenitic stainless steels also have a defect of low yield strength. For example, according to the GB 150 pressure vessel standard, one method is to use the yield strength and tensile strength of a material as reference points and divide them by their corresponding safety factors, and then take the smaller value to determine the allowable stress value of the material. Under this method, the austenitic stainless steel material would have a low value in terms of its tolerable stress, thereby unable to fully achieve the carrying capacity of the material. If a tank container is made from the austenitic stainless steel material without any treatment upon the material, then the enhanced safety requirement will require the consumption of more stainless steel materials, which will not only increase the manufacturing and transportation costs, but also reduce the liquid storage space. This approach is very uneconomical and thus is generally not employed by manufacturers. Therefore, certain strain strengthening is performed to improve the yield strength of austenitic stainless steel materials, which allows the tank body made after the strain strengthening process to have a reduced safety requirement, thereby not only lowering the cost of production materials, but also giving the tank body better flexibility, sturdiness and yield strength. However, if any opening or aperture is made in the tank body after the strain strengthening process, the overall structural strength of the tank body will be affected, nulling the effect of strain strengthening and thus rendering the tank body no longer usable. Accordingly, once the tank body is in use, it cannot be re-opened or made a hole therein for purposes of servicing or other operations. As a result, it is critical for the auxiliary devices arranged inside the storage tank to have great stability, especially those tanks used for transportation.
Specifically, for those tanks in a liquefied natural gas vehicle for transportation of natural gas, there is a much higher requirement for the structural stability inside the tank. When an existing tank is used to fill a vehicular gas cylinder with liquid, it requires a certain safety space on the top of the liner of the gas cylinder, as a result of which an anti-overfilling device needs to be arranged inside the liner of the tank.
To that end, most existing technologies use a buffer tank inside the liner of the tank. However, the buffer tank has a bulky structure and low accuracy and therefore is not very effective in terms of controlling the filling of a non-empty tank. Some may use a floating ball valve for purposes of controlling the filling.
Floating ball valves are commonly used in our daily lives. The patent publication number CN101561057A discloses a floating ball valve, which is arranged outside the valve and connected with the internal valve via a lever so that the valve can open and close through the buoyancy of the floating ball in the liquid. Since the floating ball is arranged outside the floating ball valve, however, the floating ball valve is very unstable if used in the transportation process of the tank body, and the valve assembly is prone to wear and then becomes ineffective. In order to enable the floating ball valve to be used in liquefied gas transportation tanks, the stability of the floating ball filling-control valve needs to be guaranteed and the wear of the floating ball filling-control valve need to be alleviated. To this end, those skilled in the art decided to directly dispose the floating ball in the liquid feeding pipe so that it is constrained by the inner walls of the liquid feeding pipe and thus unable to move freely. This approach increases the stability of the floating ball, but causes new problems, one of which is, the diameter of the liquid feeding pipe cannot be too large because the liquid feeding pipe is fixed inside the tank body, and therefore the size of the floating ball is limited, which generally results in failure of the floating ball and the filling-control valve. In addition, the floating ball may not fit tight within the inner diameter of the wall of the liquid feeding pipe, and as such, the stability of the floating ball cannot be guaranteed even though the floating ball is arranged in the liquid feeding pipe. Once the filling-control valve fails to function, the tank body is filled up with cryogenic liquid beyond a limit, which can cause an excessively high pressure to the tank body to make the tank body highly susceptible to explosion.
Therefore, although the above approach, by directly disposing the floating ball filling-control device inside the liquid feeding pipe, can control the movement of the floating ball to some extent, the floating ball filling-control valve is still very unstable in the transportation process of the tank body because the size of the floating ball is limited and the ball is not completely stabilized. As a result, the safety in transportation of the tank is severely unpredicable under this approach. Furthermore, due to the instability of the floating ball filling-control valve, the frequency of damages to this valve is increased. Even if only a part of the filling-control valve is damaged, the tank will be no longer usable once the floating ball filling-control valve is damaged. This is because the tank cannot be re-opened or made a hole therein for replacement or servicing purposes. Therefore, a need exists for those skilled in the art to figure out how to extend the service life of the tank and provide stability of the filling-control device.